1. Field of the Invention
The present invention relates to soft contact lenses made of a hydrated polymer or hydrogel and methods of forming such lenses.
2. Description of the Prior Art
Soft contact lenses must have a high level of oxygen transmissibility so as to avoid any adverse effect on the eyes of the wearer through insufficient oxygen being supplied to the cornea. Present lenses rely on using materials which are permeable to gases such as oxygen and carbon dioxide, since the cornea is avascular and acquires oxygen directly from the atmosphere in order to respire. Soft or hydrogel lenses are formed from polymers which when hydrated swell to a soft jelly like consistency and can be worn comfortably on the eye. It is known that the oxygen permeability of a hydrated contact lens is related to its equilibrium water content. A high water content is therefore desirable as the "dissolved" oxygen permeability of hydrogels increases almost exponentially with increasing water content up to a limiting value equal to the oxygen permeability of water. Much work has been devoted to developing polymeric materials which will form hydrogels with a high water content yet be sufficiently strong to withstand the physical stresses which will inevitably occur as the lens is handled by the wearer in placing or removing the lens from the eye. The transfer of oxygen and other gaseous species through known lenses is related to the fall in oxygen concentration from the lens surface in contact with the atmosphere to the concentration at the surface in contact with the eyes.
As stated above, the dissolved oxygen permeability of a hydrogel material is related to the water content of the lens. In turn the oxygen flux (F) through the lens into the corneal epithelium required for any contact lens thickness (L) is related to the difference (.DELTA.p) of oxygen tension across the lens and to the dissolved oxygen permeability (Pd) of the material forming the lens. F may be expressed as: ##EQU1##
The transfer of oxygen to the corneal epithelium is a complex physiological process and while calculations can be made of oxygen flux demand, these can be based on an over-simplification of the real in vivo situation which occurs when the lens is in place on the eye. Measurements are difficult as the oxygen requirements of one person can differ from those of another. in fact there are groups of people who find existing soft contact lenses impossible to wear because there is insufficient transfer of oxygen. Many papers discuss the requirements for "dissolved oxygen permeability" and in one such paper Ng and Tighe (British Polymer journal December 1976 page 118 to 123), apart from discussing the design requirements of hydrogel materials to be used for contact lenses, point out a number of factors which can complicate a theoretical approach to the problem of oxygen transfer namely: "(a) the equilibrium water content of a hydrogel contact lens, such as Hydron or Bionite, has been found to be lower in the eye than in the saline soaking solution, which is in turn lower than in distilled water. (b) The evaporation of water from the anterior surface of the contact lens during wear might result in the back flow of water from the layer of the tear fluid between the contact lens and the cornea, and hence a reduction in the oxygen tension at the epithelial surface. (c) Because of the presence of solutes in the tear fluid, the solubility of oxygen in the tear fluid may be lower than that in distilled water. (d) Because of the presence of solutes in the tear fluid, again, the structure of water in the hydrogel contact lens worn in the eye may not be the same as in distilled water and so may affect the oxygen permeability. (e) The oxygen consumption rate of the human cornea varies from one person to another and is not constant for a given individual. (f) Under closed-eye conditions, the eye-ball movement may contribute to tear fluid replenishment behind the lens.
A calculation of minimum oxygen flux for particular requirements can be made, and general guidance given that such a flux is likely to be attained or exceeded at the temperature of the eye with lens materials having particular levels of oxygen permeability. The paper quoted suggests a water content of 65% at 34.degree. C. "provides a reasonable basis for meeting problems of manufacture and visual stability with the oxygen consumption requirements of the cornea over successive day and night cycles."
One of the complications referred to is the fact that at certain levels of loss of water from the front surface of the lens there can be a back flow of water from the cornea contacting surface of the lens thereby causing a reduction of oxygen tension at the epithelial surface. This is a recognised problem which has been mentioned by several authors as having a limiting effect on the spread of the use of soft hydrogel contact lenses.
All soft contact lenses available at present have virtually the same water content at both major surfaces of the lens. Once placed in the eye, there is a drop in water content but this occurs in a uniform manner so that the content at both surfaces remains the same. However, if due to climatic conditions or any other reason there is a loss of water from the front surface of the lens, this may be sufficient to reduce or reverse the transfer of oxygen to the cornea.
Oxygen permeability (DK) is an intrinsic property of the materials used for hydrogel lenses, and the limiting factor to date in increasing the permeability of the hydrogel has been the fact that the maximum level will be that of the DK of pure water. At present if one has two lenses, namely a thick lens and a thin lens made of the same hydrogel material, the thin lens will have a higher oxygen transmissibility (DK/.sub.L), i.e. it will allow greater amounts of oxygen to pass through, but there are limits to how thin one can make a high water content lens and still have a sufficiently robust lens to withstand the stresses of even one insertion in the eye.